Directional control has become increasingly important in the drilling of subterranean oil and gas wells, with a significant proportion of current drilling activity involving the drilling of deviated boreholes. Such deviated boreholes often have complex profiles, including multiple doglegs and a horizontal section that may be guided through thin, fault bearing strata, and are typically utilized to more fully exploit hydrocarbon reservoirs.
Deviated boreholes are often drilled using downhole steering tools, such as two-dimensional and three-dimensional rotary steerable tools. Certain rotary steerable tools make use of a plurality of independently operable blades that are disposed to extend radially outward from a blade housing into contact with the borehole wall. The direction of drilling may be controlled, for example, by controlling the magnitude and direction of the force on the blades or the magnitude and direction of the displacement applied to the borehole wall. In such rotary steerable tools, the blade housing is typically deployed about a rotatable shaft, which is coupled to the drill string and disposed to transfer weight and torque from the surface (or from a mud motor) through the steering tool to the drill bit assembly. Other rotary steerable tools are known that utilize an internal steering mechanism and therefore don't require blades (e.g., the Schlumberger PowerDrive rotary steerable tools).
Rotary steerable blades are commonly actuated via electronically controlled hydraulic mechanisms. For example, U.S. Pat. Nos. 5,168,941 and 6,609,579 to Krueger et al disclose rotary steerable tool deployments in which the direction of drilling is controlled by controlling the magnitude and direction of a side (lateral) force applied to the drill bit. The amount of force on each blade is controlled by controlling a hydraulic pressure at the blade, which is in turn controlled by proportional hydraulics or by switching to the maximum pressure with a controlled duty cycle. An alternative hydraulic actuation mechanism is further disclosed in which each steering blade is independently controlled by a corresponding hydraulic piston pump. During drilling each of the piston pumps is operated continuously via rotation of a drive shaft. A control valve positioned between each piston pump and its corresponding blade controls the flow of hydraulic fluid from the pump to the blade.
U.S. Pat. No. 5,603,386 to Webster discloses another example of a rotary steerable tool employing electronic control of hydraulic blade actuation. Webster discloses a mechanism in which the direction of drilling is controlled via controlling the radial position of the blades. A hydraulic mechanism is disclosed in which all three blades are controlled via a single pump and pressure reservoir and a plurality of valves. In particular, each blade is controlled by three check valves. The nine check valves are in turn controlled by eight solenoid controlled pilot valves. Commonly assigned U.S. Pat. No. 7,204,325 to Song et al employs hydraulic actuation to extend the blades and a spring biased mechanism to retract the blades. Spring biased retraction of the blades advantageously reduces the number of valves required to control the blades, however, a significant number of controllable components are still required.
The above-described prior art steering tools employ complex electronic circuitry in order to control the hydraulic actuation of the blades. This electronic circuitry is deployed in a common housing with the hydraulic control mechanism and the blades. While such tool deployments are known to be commercially serviceable, there is room for further improvement. For example, deployment of the electronic circuitry and the hydraulic components in a common housing tends to complicate tool assembly procedures (especially in small diameter “slim” tools). Moreover, disassembly of the entire tool is commonly required when problems are identified during assembly or testing of the tool. Such disassembly and the subsequent reassembly are time consuming and expensive. Owing to the demand for smaller diameter and less expensive rotary steerable tools, there is a need for further improvement.